Two types of power supply control circuits are constant on time (COT) circuits and ramp pulse modulation (RPM) circuits. A COT circuit operates at a relatively fixed frequency without a clock. Typically, the COT circuit controls a switch such that its on-time varies inversely with the input voltage. The COT circuit initiates an on time period when the feedback voltage falls below a reference voltage. The switch remains on for a programmed on-time, causing the feedback voltage to rise above the reference voltage. After the on-time period, the switch remains off until the feedback voltage falls below the reference voltage.
An RPM circuit uses a ramp signal, such as a triangular or sawtooth waveform, that is compared to an error signal. The result of the comparison controls the pulse widths of pulses within an output signal, creating a duty signal for controlling a gate driver. An example of such an RPM circuit is described below with respect to FIG. 1.
While RPM circuits typically provide more accurate output regulation and less overshoot during load step-down transient events than COT circuits, conventional RPM circuits experience large variations in switching frequency because the ripple and perturbation in the output signal of the error amplifier result in variation of the on-time. Moreover, in the steady-state operation, jitter is present in frequency and in pulse width. Such jitter can introduce power supply variations, which can impact performance of associated circuitry.
In the following description, the use of the same reference numerals in different drawings indicates similar or identical items.